Materials that change color in response to external stimuli are known as “chromic materials”. Such chromic materials may radiate, lose color, or change properties induced by external stimuli. Different stimuli result in different responses in the material being affected.
Polydiacetylenes (“PDAs”) are a series of conjugated polymers which can undergo thermochromic transitions when exposed to temperature stimuli. PDAs have a one-dimensional conjugated backbone with a strong π to π* absorption band in the red spectral region of the optical spectrum which gives rise to an intense blue color in the polymer. The blue phase undergoes a temperature-induced or thermochromic transition observed in many PDAs to a red phase on heating. The blue to red chromatic transition is either irreversible or reversible under heating and cooling cycles depending on the chemical structure and interactions on the side chains of the PDA. In the blue phase, the strain induced by hydrogen bonding at the head groups leads to an increase in π-electron conjugation length. However, when hydrogen bonding interactions are disrupted by heat, the side group strain is released leading to twisting of the π-electron orbitals, decrease of π-electron conjugation and concomitant transition to a red phase. The red phase can rapidly reverse back to the blue phase on cooling when interactions due to: (a) strong head aromatic groups, (b) ionic moieties, and (c) covalent bond, enhanced hydrogen and multibonding bonding at the head groups are present in the PDA structures. The red phase is irreversible when the head group interactions cannot be restored on cooling. These PDAs are therefore either irreversible or reversible sensors.
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